1. Field of the Invention
The present invention relates to a signature circuit and a method of reading signature information, and more particularly to a signature circuit and a method of reading signature information that is stored in a semiconductor device.
2. Description of the Related Art
Manufacturing information of a semiconductor memory device typically includes mask information, fuse information, wafer processing information, package assembling and test information, and the like. The manufacturing information is traditionally written into a semiconductor chip using a fuse program circuit. The fuse program circuit is commonly referred to as a signature fuse circuit, and the signature fuse circuit is used for programming the lot number, die information such as a location on a wafer, and so on, so that this information can be used at a later time, for example to track a source of a defect.
Three methods are typically used to read the manufacturing information contained in the semiconductor memory devices.
A first method includes naked eye analysis of whether the signature fuse has been blown by decapping the semiconductor chip. The first method has a disadvantage that high costs are required for decapping the chip.
A second method includes measuring a current flowing through an input pin of the device to read the signature fuse data in an analog manner. According to the second method, the current is measured by allowing a current to flow through the signature fuse circuit in response to a test mode signal PSIGTEST, and the measured current is compared with a reference current as illustrated in FIG. 1. A problem of the second method is that it takes a long time to test, errors may occur in a variable test environment, and it is difficult to analyze the data using a low power voltage.
A third method includes digitally reading the signature fuse data by operating in a test mode. The third method is further divided into two approaches, that is, a register-read approach and a normal read approach.
In a register operation device such as Rambus dynamic random-access memory (RDRAM), the register-read method may include reading a register by adding only a signature fuse. However, when a semiconductor device is not a register operation device, an additional logic circuit is needed, so chip size and device layout area may be increased.
The normal read method includes adding a separate logic circuit as illustrated in FIG. 2. According to the normal read method, cell data IO and IOB are prevented from being read in the test mode, signature information SIG_IO and SIG_IOB are read through a cell data output path in the test mode. A problem of the normal read method is that an operating speed may be reduced due to the logic circuit that is added to load the signature information on the cell data output path, and the logic circuit that is needed for reading only the signature information can increase the overall size of the system.
In Korean Patent Publication Nos. 2004-68720 and 2004-69824, methods for digitally reading signature fuse data are disclosed.